A history of the trig pillar

Yesterday marked 80 years since the trig pillar was first used in the retriangulation of Great Britain on 18 April 1936. On that day, a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby and began the retriangulation of Great Britain.

We’re celebrating by sharing the story of the humble trig pillar, still much loved by walkers today, and giving you the chance to join our celebrations with The Trig Pillar Trail Challenge. But what is the background to the trig pillar?

What was the retriangulation of Great Britain?

Triangulation is a mathematical process that makes accurate map making possible. In the early 20th century, map making was still based on the Principal Triangulation which was a piecemeal collection of observations taken between 1783 and 1853. The system was starting to collapse and couldn’t support the more accurate mapping needed to track the rapid development of Britain going on after the Great War.

A diagram we found in the OS archives of the mountains from which readings were taken when surveying Ben Nevis

In 1935 Ordnance Survey, in a project led by Brigadier Martin Hotine, decided to implement a complete new control network for the whole country and at the same time unify the mapping from local county projections onto a single national datum, projection and reference system. This lead to the OSGB36 datum and The National Grid, both of which are still with us today.

Who designed the trig pillar and how did it help with the retriangulation?

The man responsible for the trig pillar that we all recognise today was Brigadier Martin Hotine. Born in 1898 in Wandsworth, London, Hotine became head of the Trigonometrical and Levelling Division at OS. The Brigadier was responsible for the design, planning and implementation of the retriangulation and he designed the iconic trig pillar. He designed them to provide a solid base for the theodolites used by the survey teams to improve the accuracy of the readings obtained. As a result, they are sometimes referred to as ‘Hotine Pillars’.

Hotine, second right, planning the retriangulation

If you haven’t come across it before, triangulation works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline and in this case, those known points were the 6,500 + trig pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the retriangulation, many rounds of angles would have been measured with the observations taking several hours. But time and technologies have moved on enormously to the point where the traditional trig pillar is now obsolete in its original guise. We’ll be talking about how we map Britain in 2016 on the blog tomorrow.

You can hear more about surveying using trig pillars in this video.

A testament to surveyors of the past

Although 6,500+ trig pillars were built, hundreds have been lost to housing developments, farming, coastal erosion and other causes. The vast majority follow the standard Hotine design, but some are stone built, and in Scotland there are some ‘Vanessas’ which are taller, cylindrical concrete pillars.

A ‘Vanessa’ trig pillar on the Isle of Skye, photographed by Scott MacLucas-Paton

You can only imagine how hard it was for surveyors of the past to not only map Britain, but to also locate sites for trig pillars and carry the materials to remote sites to then build the trig pillars too.

Setting a trig pillar

Carrying materials up Sca Fell

It’s a true testament to their skills that such an accurate map of Britain was created from such humble beginnings as the trig pillar 80 years ago.

Find out more and get involved in the #TrigPillar80 celebration

Read yesterday’s blog about the 80th birthday celebrations, and join us tomorrow to find out how we survey and map Britain today.

Take a look at our Flickr album with trig pillar photos past and present.

Take part in The Trig Pillar Trail Challenge and share your trig pillar photos with us using #TrigPillar80 on Twitter and Instagram – and you could win a limited edition T-shirt.

Our first trig pillar was over in Cold Ashby, Northamptonshire and has a plaque on there. It was first used in the retriangulation of Great Britain on 18 April 1936 – so that’s why we’ve been celebrating the 80th anniversary of the trig pillar this year. Hope that helps.

I have been trying to find out if there are any books on the OS and trig pillars. Would you happen to know if there are? And if not I think it would make a great guide book for those trig-baggers out there if anyone feels disposed to write one.

How was the trig pillar used in measurements? I understand the principle of triangulation but the datum point was presumably at ground level whereas the theodolite was a finite (and constant) height above this. Was there a simple constant used in obtaining ground heights?

Looking at the cross sectional diagram of a pillar will help with this explanation from our Geodetics team.

The lower bolt was so the point could easily recreated if the pillar and top bolt were destroyed. The top bolt (roughly ground level in centre of pillar) was the mark that was being coordinated when the pillar was observed from or to. During the construction of the pillar the spider (three legged brass “device”) which holds a theodolite on the top of the pillar was centred over the top bolt so a theodolite placed on the spider was automatically centred over the bolt.

Nearly every pillar has a flush bracket on the side which is the (above mean sea level) height reference mark for the pillar. Due to variations in pillar construction a constant height from FB to pillar top cannot be guaranteed so the height of an instrument during a survey would always be measured from the FB. This ties the survey observations into height above mean sea level.

one of the saddest things is the very first trig pillar is inaccessible.
It’s about 50 meters in on some farm land and there is no access to this bit of history. The farmer even parks a trailer in front of it so you can’t see it. Real shame as it was such a significant development.

Stuart, thank-you for your question. Pinpointing an exact year is hard unfortunately. Using the traditional survey techniques for which they were designed (using a theodolite), trig pillars have been redundant since the mid 90s. Once used with GPS, trig pillars have been fully redundant from all uses since roughly 2001. I hope this helps, Jocelyn

Trig pillars may be redundant for OS surveying purposes, but over 300 of them, along with nearly 700 other triangulation stations (surface blocks, bolts, FBMs, FBM auxiliaries etc) form the OS’s Passive Network which is supposedly still maintained by the OS and is available to third-party surveyors as and when needed.

Hi Stuart.
Levelling Networks are never given an accuracy relative to a single point/datum, instead they are expressed as an accuracy relative to distance levelled for example ±2.0√km mm. Like all other traditional control networks, the levelling network is broken down into smaller parts of decreasing accuracy and increasing density. So, at the top of the hierarchy we have the approx. 200 Fundamental Bench Marks (FBMs) which are very accurate and can be considered as “zero order” and almost error free. The FBMs still form the realisation of Ordnance Datum Newlyn (ODN) – their heights are a crucial input to the OSGM15 geoid model that models the relationship between ODN and heights from GPS receivers.
The relative accuracy of lower order benchmarks, including the familiar cut marks, can be expressed as a function of the distance levelled. Details are given in the “Benchmarks” section on this web page which details the accuracy of our legacy control data sets – https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/accuracy-legacy-data.html
Hope this helps, Jocelyn

John,
The instrument testing scales look to be an instrument to check the horizontal scale on a theodolite (ie for measuring angles not weight) but we cannot be certain, my colleague I spoke to says he has not used anything similar. The three equidistant grooves look like the top of a pillar and would ‘force centre’ a tribrach, but it doesn’t in itself have anything to do with trigs. Hope this helps, Jocelyn

So a check horizontal scale to check the theodolite’s horizontal rotational movement. That’s interesting. Thank you for the explanation. I bought a periscope sextant from the same place. I do know what that does so I have a chance of using that. Think I will leave the instrument testing scale alone.

Hi.
Can you please explaine the function of the four small recesses in the flush
bracket above the bench mark arrow. I have an old and vague recollection
of them locating a crab like device which then protrudes away from the pillar
at the height of the bench mark. Is this correct?

Yes, you’re right, the recesses were designed to support a bracket which was used when the benchmark was being observed. Obviously, these haven’t been used for decades now and we use modern GNSS equipment for surveying today.

Good afternoon, i am a member of the Cunningsburgh History Group In the Shetland Islands, there is a trig point at Helliness Cunningsburgh that over looks the sea where on the 30th June 1917 a destroyer HMS Cheerful that was escorting a convoy of nine ships hit a contact mine she broke in two and sunk, 44 men lost their lives, this event has more or less been forgotten about, we did remember them on the 100th anniversary and led a wreath at the trig point, we intend to research this as a project try and get contact with decendents of these men, and we would like to remember these men and thought if we could commission a plaque with the names of these men and the story of the ship and wondered if it was possible to erect the plaques on your trig point, this would be a nice way to reuse your trig point and would be a focal point for people and decendents to come and pay there respects to this tragic event that happened over 100 year ago, thanks for your time and hope to hear from you Ian.

Hi Ian, thanks for your comment. We have no problem with a plaque as long as it does not interfere with the top plate of the pillar or the flush bracket on the side. We recommend using exterior grade adhesive to fix to the pillar rather than bolts. This will reduce the risk of water ingress to the concrete and eventual frost fracturing. You will need land owner permission to access the pillar to carry out the work and we recommend you also check with them that they are okay with more people being likely to want to visit the pillar once the plaque is attached. Hope this helps, Jocelyn

Hello, I have recently started working in a 1960’s housing estate in Birmingham and was thrilled to find a trig point while exploring the other day. Would you have any records as to when it was installed etc or what number it was? It’s approx 52.404862 -1.893424
Many thanks

Hi Jayne. Based on the lat and long you gave, my colleague is pretty sure you’re referring to the “Bells Lane” pillar at map ref SP073785. If you search km square reference SP0778 on our website’s trig locator tool (https://www.ordnancesurvey.co.uk/gps/legacy-control-information/triangulation-stations), that will provide some information. We don’t hold accessible info on when it was built, but my colleague has advised me it looks like an early-ish one as it was computed in 1944. Hope this helps, Jocelyn

Christian, thank-you for your question. OS has used a wide variety of theodolites in their duties. The exact make and model is not important if the only requirement is to ‘fit’ a trig pillar. A trig pillar adaptor is used to fit between the theodolite and the three metal groves set into the top of the trig pillar (the ‘spider’). This adaptor consists of a metal plate with three pins on the underside which locate into the grooves on the trig pillar, and a 5/8” survey thread in the centre which screws into the base of most theodolites.
You can purchase a trig pillar adaptor from survey supply companies but these can be quite expensive. Those used by the OS are quite simple and could be quite easily made (a circular plate of metal with three legs set at 120 degrees around the bottom edge, and a 5/8” survey thread in the centre pointing upwards).
We may be able to source a photo too if you are interested, but unfortunately we cannot upload one here in a comment. If you would like a photo, please email me at joss.harris@os.uk. Many thanks, Jocelyn

Hi, what do the numbers on the trig point refer to? The one nearest to me has S4014 on it. Is this just its number of the 6500 constructed or does it refer to something else?
It is in Sompting, West Sussex.
Many thanks in advance.

Hi Geoff, good question! S4014 is the flush bracket number. The numbers are simply a unique identifying number cast into the bracket when it was made. There is no direct relationship between the number and the location, ie number S4014 would have been the 4014th cast in the ‘S’ series of flush brackets but not necessarily (and almost certainly) not the 4014th trig pillar built. Some flush brackets are prefixed with a ‘G’ and these tend to be first order Geodetic Levelling monuments. Most trig pillars (but not all) are prefixed with an ‘S’. Some flush brackets have only a number cast into it with no prefix letter. I hope this helps, Jocelyn

Your email address will not be published. Required fields are marked *

Name* :

Email* :

Website:

About Us

OS is Britain's mapping agency. We make the most up-to-date and accurate maps of the country. But we're also a digital business, and we use our content to help governments, companies and individuals to be more effective both here and around the world.